From IFWiki

On some operating systems, the term "side-loading" refers to installing an application without the use of an official "store" or install app. It is common practice on Android devices to side-load APK files when working with pre-release or custom-built apps. It allows software authors to publish without all the associated marketing images, descriptions, screening and fees that normally go along with an official "store" listing. Instructions about side-loading exist on YouTube, blogs, and other help sites of the Internet.